Computer disk drives typically incorporate retrieval and storage of data by use of magnetic storage disks and read/write heads that are capable of reading data from and writing data onto the rotating storage disks. Data is stored on each magnetic storage disk in a number of concentric tracks on the disk. The narrower the tracks can be made, the more data that can be stored on the storage disk. The read/write heads may also be referred to as the read/write transducers that are integrated within a slider that typically places the heads at a predetermined height above the corresponding storage disk. One or more read/write heads may be integrated within a single slider. A suspension assembly supports the slider over the disk and maintains the slider over the desired data track center line during a read or write operation. A cushion of air is generated between the slider and the rotating disk, the cushion often referred to as an air bearing. The suspension assembly is part of the actuator that is the component in the disk drive for positioning the read/write heads. The actuator is typically controlled by a voice coil motor that acts as a primary actuator for positioning of the slider over the desired track. Because of the trend in recent years to provide greater storage capacity on a storage disk, track widths have become increasingly narrower which makes it more difficult for the read/write heads to accurately read and write information to and from the magnetic disks. The actuator has limited ability to accurately position a slider across the data tracks. Accordingly, a need has arisen over the years for the ability to more accurately position the read/write heads on tracks of decreasing width. As track density increases, the speed or servo bandwidth with which an actuator can respond must also increase to allow effective track following.
One approach to achieving finer positioning of the actuator is to employ secondary actuation that operates together with primary actuation provided by the voice coil motor. Secondary actuation can be provided in the form of an additional actuator element that enables enhanced control of the flexure and/or load beam. These additional control elements are often referred to as “micro” or “milli” actuators.
It is well known that a shock event resulting in actuator displacement can cause a read/write error. Because of the increased track densities found on most disks, inadvertent displacement of the actuator for any reason may have a greater deleterious effect since more tracks may be affected by the inadvertent displacement.
Rapid off track (ROT) events may be generally characterized as unpredictable and sporadic displacements of an actuator assembly, but not caused by a shock event in which there may be contact of the slider with the disk. One example of a ROT event includes movement of the actuator assembly due to spontaneous stress relief of one or more elements within the actuator assembly. Another example of a ROT event includes inadvertent movements created by transients in the voice coil motor power supply. More specifically, the transients in the power supply create an electromagnetic force (EMF) on the yoke that carries the voice coil, thus resulting in inadvertent and unpredictable actuator movement.
While the amplitude of a ROT event may be unpredictable, through observation and testing, it has been found that the great majority of all ROT events occur within a one millisecond duration and further, such ROT events have a frequency of between about 10-30 kilohertz. As compared to shock events, ROT events are of a much higher frequency and of a significantly shorter duration. For example, a shock event resulting in contact of read/write heads against the disk typically has a duration of at least 3-4 milliseconds, and the frequency is typically between about 1-3 kilohertz. Additionally, through observation it has been found that ROT events cause lateral actuator displacement with negligible vertical displacement, while most non-ROT events have a significant vertical displacement component.
One reference that discloses a system and method for detecting displacement of disk drive heads mounted on microactuators due to contact with disks is the U.S. Pat. No. 6,600,622. More specifically, this reference discloses a disk drive with a detector circuit connected to a distal end of a two-stage actuator. The actuator includes a microactuator for fine track positioning of read/write heads relative to a disk. Intermittent contact between the head and the disk produces forces that are detected and measured by the microactuator drive circuitry. The measurements are used to determine if excessive contact is occurring between the head and the disk. A signal from the microactuator is used to electronically detect slider/disk contact. The microactuator may be piezoelectric or voice coil based, and capable of generating a signal in response to an applied force or displacement.
While this reference may be adequate for its intended purpose, there is still a need for detection and remedial actions to be taken with respect to ROT events that are more difficult to detect than shock events. Further, there is a need for remedial actions to be taken including sequential analysis of surrounding disk tracks to confirm the status of data on tracks potentially affected by the ROT event.